From 3d6ac9037deb2d3f042023719caf906cc873f6c9 Mon Sep 17 00:00:00 2001
From: Nana Sakisaka <1901813+saki7@users.noreply.github.com>
Date: Sat, 14 Feb 2026 20:41:19 +0900
Subject: [PATCH] Improve error diagnostics

---
 include/iris/x4/core/action.hpp          |  8 +++++++-
 include/iris/x4/core/skip_over.hpp       | 19 +++++++++++++++++++
 include/iris/x4/directive/skip.hpp       | 19 +++++++++++++++++++
 include/iris/x4/operator/alternative.hpp | 10 ++++++++++
 include/iris/x4/operator/list.hpp        | 10 ++++++++++
 include/iris/x4/operator/sequence.hpp    | 22 ++++++++++++++++++++++
 6 files changed, 87 insertions(+), 1 deletion(-)

diff --git a/include/iris/x4/core/action.hpp b/include/iris/x4/core/action.hpp
index aeb90c3e6..f5aa6c5bf 100644
--- a/include/iris/x4/core/action.hpp
+++ b/include/iris/x4/core/action.hpp
@@ -17,11 +17,12 @@
 #include <iris/x4/core/context.hpp>
 #include <iris/x4/core/action_context.hpp>
 
-#include <ranges>
+#include <ranges> // subrange
 #include <iterator>
 #include <concepts>
 #include <type_traits>
 #include <utility>
+#include <format>
 
 namespace iris::x4 {
 
@@ -109,6 +110,11 @@ struct action : proxy_parser<Subject, action<Subject, ActionF>>
 
     constexpr void operator[](auto const&) const = delete; // You can't add semantic action for semantic action
 
+    [[nodiscard]] constexpr std::string get_x4_info() const
+    {
+        return std::format("{}[f]", get_info<Subject>{}(this->subject));
+    }
+
 private:
     // Semantic action with no parameter: `p[([] { /* ... */ })]`
     template<class Context, X4Attribute Attr>
diff --git a/include/iris/x4/core/skip_over.hpp b/include/iris/x4/core/skip_over.hpp
index d312c6f95..511820e64 100644
--- a/include/iris/x4/core/skip_over.hpp
+++ b/include/iris/x4/core/skip_over.hpp
@@ -42,6 +42,25 @@ enum struct builtin_skipper_kind : char
     space,
 };
 
+namespace detail {
+
+template<builtin_skipper_kind Kind>
+struct builtin_skipper_traits;
+
+template<>
+struct builtin_skipper_traits<builtin_skipper_kind::blank>
+{
+    static constexpr char const* name = "blank";
+};
+
+template<>
+struct builtin_skipper_traits<builtin_skipper_kind::space>
+{
+    static constexpr char const* name = "space";
+};
+
+} // detail
+
 template<std::forward_iterator It, std::sentinel_for<It> Se, class Context>
     requires X4Subject<get_context_plain_t<contexts::skipper, Context>>
 constexpr void skip_over(It& first, Se const& last, Context const& ctx)
diff --git a/include/iris/x4/directive/skip.hpp b/include/iris/x4/directive/skip.hpp
index 33c594e3d..29aa8cde6 100644
--- a/include/iris/x4/directive/skip.hpp
+++ b/include/iris/x4/directive/skip.hpp
@@ -17,6 +17,7 @@
 
 #include <iris/x4/char/char_class_tags.hpp>
 
+#include <format>
 #include <concepts>
 #include <iterator>
 #include <type_traits>
@@ -49,6 +50,15 @@ struct skip_directive : proxy_parser<Subject, skip_directive<Subject, Skipper>>
         return this->subject.parse(first, last, x4::replace_first_context<contexts::skipper>(ctx, skipper_), attr);
     }
 
+    [[nodiscard]] constexpr std::string get_x4_info() const
+    {
+        return std::format(
+            "skip({})[{}]",
+            get_info<Skipper>{}(this->skipper_),
+            get_info<Subject>{}(this->subject)
+        );
+    }
+
 private:
     template<class Context>
     using context_t = std::remove_cvref_t<decltype(
@@ -98,6 +108,15 @@ struct builtin_skip_directive : proxy_parser<Subject, builtin_skip_directive<Kin
         /* constexpr */ builtin_skipper_kind skipper_kind = Kind;
         return this->subject.parse(first, last, x4::replace_first_context<contexts::skipper>(ctx, skipper_kind), attr);
     }
+
+    [[nodiscard]] constexpr std::string get_x4_info() const
+    {
+        return std::format(
+            "skip({})[{}]",
+            detail::builtin_skipper_traits<Kind>::name,
+            get_info<Subject>{}(this->subject)
+        );
+    }
 };
 
 
diff --git a/include/iris/x4/operator/alternative.hpp b/include/iris/x4/operator/alternative.hpp
index 84a1a323a..8c2029c29 100644
--- a/include/iris/x4/operator/alternative.hpp
+++ b/include/iris/x4/operator/alternative.hpp
@@ -21,6 +21,7 @@
 
 #include <iris/rvariant/rvariant.hpp>
 
+#include <format>
 #include <concepts>
 #include <iterator>
 #include <type_traits>
@@ -153,6 +154,15 @@ struct alternative : binary_parser<Left, Right, alternative<Left, Right>>
         }
         return false; // `attr` is untouched
     }
+
+    [[nodiscard]] constexpr std::string get_x4_info() const
+    {
+        return std::format(
+            "{} | {}",
+            get_info<Left>{}(this->left),
+            get_info<Right>{}(this->right)
+        );
+    }
 };
 
 template<X4Subject Left, X4Subject Right>
diff --git a/include/iris/x4/operator/list.hpp b/include/iris/x4/operator/list.hpp
index 1fc2589ab..8c23f0183 100644
--- a/include/iris/x4/operator/list.hpp
+++ b/include/iris/x4/operator/list.hpp
@@ -19,6 +19,7 @@
 
 #include <iris/x4/traits/container_traits.hpp>
 
+#include <format>
 #include <iterator>
 #include <type_traits>
 #include <utility>
@@ -63,6 +64,15 @@ struct list : binary_parser<Left, Right, list<Left, Right>>
             return true;
         }
     }
+
+    [[nodiscard]] constexpr std::string get_x4_info() const
+    {
+        return std::format(
+            "({} % {})",
+            get_info<Left>{}(this->left),
+            get_info<Right>{}(this->right)
+        );
+    }
 };
 
 template<X4Subject Left, X4Subject Right>
diff --git a/include/iris/x4/operator/sequence.hpp b/include/iris/x4/operator/sequence.hpp
index b2d95c11d..f6184fbd0 100644
--- a/include/iris/x4/operator/sequence.hpp
+++ b/include/iris/x4/operator/sequence.hpp
@@ -20,7 +20,9 @@
 #include <iris/x4/directive/expect.hpp>
 
 #include <iris/alloy/tuple.hpp>
+#include <iris/type_traits.hpp>
 
+#include <format>
 #include <concepts>
 #include <iterator>
 #include <type_traits>
@@ -28,6 +30,9 @@
 
 namespace iris::x4 {
 
+template<class Subject>
+struct expect_directive;
+
 template<class Left, class Right>
 struct sequence : binary_parser<Left, Right, sequence<Left, Right>>
 {
@@ -74,6 +79,23 @@ struct sequence : binary_parser<Left, Right, sequence<Left, Right>>
     {
         return detail::parse_sequence(*this, first, last, ctx, attr);
     }
+
+    [[nodiscard]] constexpr std::string get_x4_info() const
+    {
+        if constexpr (iris::is_ttp_specialization_of_v<Right, expect_directive>) {
+            return std::format(
+                "{} > {}",
+                get_info<Left>{}(this->left),
+                get_info<typename Right::subject_type>{}(this->right.subject)
+            );
+        } else {
+            return std::format(
+                "{} >> {}",
+                get_info<Left>{}(this->left),
+                get_info<Right>{}(this->right)
+            );
+        }
+    }
 };
 
 template<X4Subject Left, X4Subject Right>